Recovery of oxygen from the atmosphere



March 28, 19 50 w. DENNIS 2,502,250

RECOVERY OF OXYGEN FROM THE ATMOSPHERE Filed Dec. 15, 1946 INVENTOR flfw'k'az? Franz:

v BY

ATTORNEYS Patented Mar. 28, 1950 RECOVERY OF OXYGEN FROM THE ATMOSPHERE Wolcott Dennis, Darien, Conn., assignor to Air Reduction Company, Incorporated, New York, N. Y., a corporation of New York Application December 13, 1946, Serial No. 715,929

11 Claims. (c1. 62-1755) This invention relates to the separation of oxygen and nitrogen from air by liquefaction and rectification, and particularly to improvements permitting the recovery of both high and low purity oxygen simultaneously and economically, especially in large scale operations adapted to produce oxygen for industrial uses.

There are numerous potential industrial uses for oxygen produced at relatively low cost. Heretofore, the cost of production has prevented many of these uses. The oxygen need not be of ordinary commercial purity, i. e. 99.5% or better, for many applications. A purity of 90% more or less is usually acceptable. The oxygen must, however, be produced in volume far exceeding the output of the present commercial units, entailing, therefore, the use of apparatus of much greater capacity. Since, however, high purity oxygen such as the present commercial product is generally in demand, it is desirable that both high and low purity oxygen products be available from the same operation.

The handling and treatment of the large volume of air sufficient to afford 100 to 1,000 tons of oxygen daily involves numerous problems other than the mere capacity of the apparatus, although all units must necessarily be enlarged substantially in order to afford economical operation. One of the principal problems is the removal of carbon dioxide and hydrocarbons present in minute amounts in the atmosphere with the minimum of chemical treatment.

It is the object of the present invention to provide an economical and commercially practicable procedure for the treatment of relatively large volumes of air and the recovery therefrom of both high and low purity oxygen simultaneously.

Another object of the invention is the provision of a procedure which eliminates carbon dioxide and hydrocarbons from the bulk of the air treated at the initial stage of the operation, thus avoiding contamination of the oxygen product with these impurities.

Other objects and advantages of the invention will be apparent as it is better understood by reference to the following specification and the accompanying drawing, which illustrates diagrammatically an apparatus suitable for the practice of the invention, features well known in the art being omitted for purpose of clarity.

Referring to the drawing, the bulk of the entering air, after compression to a suitable pressure, for example approximately -6 atmospheres, and passage through the usual intercoolers (not shown), is delivered by a pipe 5 and branches 6 and 1 to regenerators 8 and 9 in which material is brought to low temperature by heat exchange with cold outgoing products and thereafter the incoming air is cooled by contact with the cold material. The regenerators are arranged so that one is being cooled while the other is utilized for cooling the air. Any necessary number of regenerator units may be used. The details of regenerators are well known in the art, and further description thereof is accordingly unnecessary.

The air is delivered through pipes I0 and II to a pipe I2 which is connected to a condenser and rectifier section I3 at the bottom of the column II. The air proceeds upwardly through one or more trays I5, having the usual caps l6, into contact with a condenser coil I1 horizontally disposed and cooled by the cold nitrogen eilluent from the column H in the manner hereinafter described. The initial condensation produces a liquid which washes the air rising through the vtray I5, producing thereby a relatively small The liquid thus freed from impurities joins the expanded portion of the purified air escaping from the section l3 through a pipe 22 to an ex pander, preferably a turbine 23. From the turbine, the air is delivered at the lower pres sure through a pipe 24 to which the pipe 2| is connected, into the column H.

The air entering the column i4 is augmented by an additional portion of air previously freed from carbon dioxide and hydrocarbons by con-' tact with suitable purifying agents in the manner well known in the art. This air constitutes only a minor portion of the air entering the system. It is initially compressed to a pressure of approximately 5-6 atmospheres, cooled and then delivered through a pipe 25 to an exchanger 28 where it is further cooled by heat exchange with the liquid oxygen product of the operation supplied through a pipe 21 and pump 28. The liquid oxygen is vaporized and escapes through a pipe 29. Alternatively, gaseous oxygen may be withdrawn, in which case the pump is omitted. This The liquid constitutes the pure oxygen product of the operation.

The amount of auxiliary air entering the exchanger 26 is greater in mass than the high purity oxygen product by an amount equivalent to 3-4% 'of the total air flow to the plant, this excess eventually passing out through the low purity oxygen outlet and with the waste nitrogen effluent from the operation to preserve the proper thermal relations for complete removal of carbon dioxide and other impurities from the regenerators 8 and 9. Thus, the auxiliaryair constitutes usually from 5 to 8% of the total air entering the system. This air, after leaving the exchanger 29, passes through a pipe 30 to the pipe 22 and thence through theturbine 23.

A portion of the air treated in the section I3 is withdrawn through a pipe 3| and is delivered to a high pressure rectification column 32 having the usual trays 33 and caps 34. The air passes upwardly through the trays in contact with nitrogen reflux liquid provided as hereinafter described. The column operates at the initial pressure of the entering air, preferably 5-6 atmospheres. As a result of the rectification, an oxygen-enriched liquid accumulates in the bottom of the column. It is withdrawn through a pipe 35 and pressure-reducing valve 36 and is delivered by a pipe 31 to the pipe 2|, thus entering the column M with the remaining portion of the air.

In the column M, which is supplied with trays 38 and caps 39, the air passes upwardly through the trays in contact with reflux liquid supplied by condensation of a part of the air at the top of the column. This condensation is efiected by heat exchange of vapors in the column with liquid supplied from the bottom of the column l4 through a pipe 40 and expansion valve 4|. The pipe 40 delivers the liquid to a horizontal condenser 42. The resulting rectification eliminates nitrogen which escapes through a pipe 43 and is delivered to the condenser coil I! in the section l3. Here the nitrogen is warmed to increase the available energy in the subsequent expansion of the nitrogen. The cooling efiect of the nitrogen is utilized to produce reflux liquid for the initial separation as hereinbefore indicated. Thence the nitrogen escapes through a pipe 44 and expander, preferably a turbine 45, to a pipe 46, exchanger 8 and outlet pipe 41.

The column l4 operates at an intermediate pressure, for example 2.5 to 3 atmospheres and, as indicated, eliminates nitrogen and produces an oxygen-enriched liquid which is delivered from the condenser coil 42 through a pipe 48 to a low pressure column 49 operating, for example, at about 1 atmosphere. ,The column 49 has trays 50 with caps 5|. The liquid supplied through the pipe 48 flows downwardly over the trays 5|] in contact with vapors rising through the column. Nitrogen refiux is provided for both the columns 32 and 49 by withdrawing the effluent from the column 32 through a pipe 52 and delivering it to a series of horizontal condenser coils 53, 54 and 55 in the bottom of the column 49. Each of the condenser coils is disposed in trays 56, 51 and 58 adapted to permit the overflow of liquid from the upper to the lower tray, thus avoiding modification of the temperature as a result of a substantial hydrostatic head of liquid.

The trays are so arranged that adequate space is allowed between the sides of the trays, which have a rectangular plan, and the column shell,

for free passage of vapors upward from lower points in the column. The pressure of the vapor space is thereby maintained at the same value over all trays and the lowest pool of liquid.

The effect of hydrostatic head may be illustrated as follows: Assume a large column, say 10 feet diameter and requiring 10 feet depth of liquid, to cover the heat transfer surface. The pressure due to hydrostatic head of liquid oxygen would be 5 p. s. i. resulting in an increase of 3 C. in the boiling point of liquid oxygen at the bottom of the pool over the boiling point at the top of the pool.

In good design, the temperature of the condensing nitrogen should be of the order of 3 C. warmer than the boiling point of the liquid oxygen at the surface of the pool. If the lower portions of the pool have a boiling point 3 C. higher than the upper portion of the pool as in the foregoing example, no temperature difference would exist between the condensing nitrogen and the boiling oxygen at the bottom of the pool. In this event the lower condenser surface would become inoperative, necessitating the use of a higher condensing pressure.

The nitrogen is liquefied in the coils 53, 54 and 55 and escapes through a pipe 59. A pump 60 delivers a portion of this liquid nitrogen through a pipe 6| to the top of the column 32 to provide the necessary reflux therein. The balance of the liquid nitrogen is delivered through a pipe 62 and valve 64 to the top of the column 49.

As a result of vaporization of liquid in the trays 56, 51 and 58, and the flow of liquid supplied through the pipes 48 and 62, rectification in the column 49 affords ellluent nitrogen which escapes through a pipe 65 connected to the pipe 46. The nitrogen is thus delivered from the system. The pure oxygen liquid accumulating in the bottom of the column 49 is withdrawn through the pipe 2'! and valve 21 and delivered by the pump 28 to the exchanger 26 as hereinbefore described. The pipe 21 may be connected above the liquid level in the bottom of the column, in which case pure gaseous oxygen is withdrawn. The pump 28 is unnecessary if gaseous oxygen is withdrawn. Thus, a substantial supply of pure oxygen, that is, 99.5% or better, is maintained.

At an intermediate level of the column 49, a pipe 66 provided with a valve 6! is connected to withdraw low purity oxygen. By adjustment of the position of the pipe 65, the purity of the oxygen withdrawn may be varied, since the proportion of oxygen difiers at different levels in the column 49. Usually the product will be or better, and it can be withdrawn in relatively large volume as compared with the gaseous volume of the pure oxygen product. The pipe 66 delivers a large volume of low purity oxygen to the exchanger 9 for heat transfer to the incoming air, and constitutes the supply of oxygen for large volume industrial uses which is the principal product of the operation.

It is necessary periodically to clean the filter I8 and to reactivate the silica gel. For that purpose, a valve 31 is disposed to shut off liquid from the compartment l3. Connections 68 and 69 with valves 10 and H permit the introduction of warm dry nitrogen to vaporize and remove the adsorbed impurities. A by-pass 12 and valve 13 are provided to maintain continuous flow of the liquid, if desired, while the filter is being cleaned.

Among the advantages of the procedure as described are the continuous production of both high and low purity oxygen, it being understood that high purity oxygen means oxygen of the usual commercial purity, and that low purity oxygen is a relative term including oxygen suitable for many industrial operations which require large volumes but not necessarily oxygen of usual commercial grade. Another advantage is the elimination of carbon dioxide, hydrocarbons, etc., in the initial part of the operation without resort to chemical means except in the case of the auxiliary air. The use of chemical purifiers in connection with the production of oxygen is well known, but the equipment is much too bulky and costly to warrant its use in connection with large volume operations such as are contemplated by the present invention. There are other advantages which will be apparent.

Various changes may be made in the details of procedure and in the apparatus employed without departing from the invention or sacrificing the advantages thereof.

I claim:

1. The method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectification of air which comprises compressing a major portion and a minor portion of the air to be treated, cooling the major portion by heat transfer to gaseous products of the separation, subjecting the major portion of the air to partial liquefaction to afford a liquid fraction containing impurities of the air, cooling the minor portion of the by heat exchange with the high purity oxygen product of the separation, combining at least a part of the unliquefied and purified major portion with the minor portion of the air, subjecting the combined fiuids to rectification to separate nitrogen in two gaseous fractions, withdrawing the greater part of the oxygen as low purity oxygen in the gaseous phase from an intermediate stage of the rectification and withdrawing the balance of the oxygen as high purity oxygen at a stage of the rectification beyond that at which the low purity oxygen is withdrawn.

2. The method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectification of air which comprises compressing a major portion and a minor portion of the air to be treated, cooling the major portion by heat transfer to gaseous products of the separation, subjecting the major portion of the air to partial liquefaction to afford a liquid fraction containing impurities of the air, rectifying a part of the major portion of the unliquefied and purified air to separate a liquid enriched in oxygen, cooling the minor portion of the air by heat exchange with the high purity oxygen product of the separation, combining the liquid enriched in oxygen with the remainder of the unliquefied and purifled air and the minor portion of the air, subjecting the combined fluids to rectification to separate nitrogen in two gaseous fractions, withdrawing the greater part of the oxygen as low purity oxygen in the gaseous phase from an intermediate stage of the rectification and withdrawing the balance of the oxygen as high purity oxygen at a stage of the rectification beyond that at which the low purity oxygen is withdrawn.

3. The method of separating the constituents of air by liquefaction and rectification which comprises compressing the major portion of the air, cooling it by heat transfer to outgoing gaseous products of the separation, separating a fraction including readily condensible impurities by partial condensation, leaving a purified residue of the major portion of the air, compressing a minor portion of the air, cooling it by heat exchange with a product of the separation, rectifying a part of the purified residue at the initial pressure to afford an oxygen enriched liquid, rectifying the latter liquid with the remainder of the purified residue and the minor portion of the air after expansion to an intermediate pressure, to afford a liquid enriched in oxygen, rectifying the latter liquid at a lower pressure to afford oxygen of low purity and oxygen of high purity, and separately withdrawing the oxygen products.

4. The method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectification which comprises compressing the major portion of the air, cooling it by heat transfer to outgoing gaseous products of the separation, compressing an auxiliary portion of the air and cooling it by heat exchange with the high purity oxygen product, subjecting a part of the major portion of the air to a first rectification at the initial pressure, subjecting the liquid product of the first rectification with the remainder of the major portion and the auxiliary portion of the air to a second rectification at an intermediate pressure, finally rectifying the liquid product of the second rectification at a lower pressure, withdrawing the low purity oxygen product from the final rectification and withdrawing the high purity oxygen product from the final rectification.

5. The method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectificat on which comprises compressing the major portion of the air, cooling it by heat transfer to outgoing gaseous products of the separation, compressing an,

auxiliary portion of the air and cooling it by heat exchange with the high purity oxygen product, subjecting a part of the major portion of the air to a first rectification at the intitial pressure, subjecting the liquid product of the first rectification with the remainder of the major portion and the auxiliary portion of the air to a second rectification at an intermediate pressure, finally rectifying the liquid product of the second rectfication at a lower pressure, withdrawing the low purity oxygen product in the gaseous phase from the final rectification and withdrawing the high purity oxygen product in the liquid phase from the final rectification.

6. In the method of separating and recovering the constituents of air, the improvement which comprises liquefying and rectifying the air to produce a liquid product enriched in oxygen, forming liquid of said liquid product into separate pools, partially vaporizing liquid of the separate pools by heat exchange with separated nitrogen, thereby liquefying the nitrogen, utilizing the liquid nitrogen as reflux in the rectification, withdrawing high purity oxygen at the lower stage of rectification and withdrawing oxygen of lower purity at an intermediate stage of the rectification.

7. In the method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectification of air, the improvement which comprises subjecting incoming air to partial condensation to separate a liquid containing impurities, filtering the liquid, combining the filtered liquid with at least a part of the purified residue of the incoming air and subjecting the combined fluids to rectification to separate nitrogen, low purity oxygen and high purity oxygen, and separately withdrawing at different stages of the rectification oxygen of low purity and oxygen of high purity.

8. In the method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectification of air, the improvement which comprises subjecting the major portion of the incoming air to partial condensation to separate a liquid containing impurities, filtering the liquid, combining the filtered liquid with at least a part of the purified residue of the major portion of the incoming air and a separate minor portion of the air, and subjecting the combined fluids to rectification to separate nitrogen, 10w purity oxygen and high purity oxygen and separately withdrawing at different stages of the rectification oxygen of low purity and oxygen of high purity.

9. In the method of separating and recovering the constituents of air, the improvement which comprises compressing and cooling the air, subjecting it to partial condensation to afford a liquid condensate containing volatile impurities of the air, filtering the condensate, combining the filtered condensate with at least a part Of the residue of the air after said partial condensation, and thereafter subjecting the combined fluids to rectification to separate nitrogen and oxygen, and withdrawing the oxygen in two fractions and at different stages of the rectification, one of high and the other of low purity.

10. In the method of separating and recovering the constituents of air, the improvement which 3,

comprises compressing and cooling the air, subjecting it to partial condensation to afford a liquid condensate containing volatile impurities of the air, filtering the condensate, combining the filtered condensate with at least a part of the residue of the air after said partial condensation, and thereafter subjecting the combined fluids to rectification to separate nitrogen and oxygen, and withdrawing the oxygen in two fractions and at different stages of the rectification, one fraction being liquid oxygen of high purity and the other fraction being gaseous oxygen of low purity.

11. The method of separating and recovering simultaneously high and relatively low purity oxygen by liquefaction and rectification of air which comprises compressing a major portion and a minor portion of the air to be treated, cooling the major portion by heat transfer to gaseous products of the separation, subjecting the major portion of the air to partial liquefaction to afford a liquid fraction containing impurities oi the air, cooling the minor portion of the airbyheat exchange with the high purity oxygen product of the separation, combining at least a part of the unliquefied and purified major portion with the minor portion of the air, subjecting the combined fluids to rectification to separate nitrogen therefrom as a gaseous fraction, thereafter rectifying the residue from which the gaseous nitrogen was separated during such rectification in a second rectification zone to separate a further amount of nitrogen as a second gaseous nitrogen fraction, withdrawing a part of the oxygen as low purity oxygen in the gaseous phase from an intermediate stage of the second rectification zone, and withdrawing the balance of the oxygen as high purity oxygen at a stage of the second rectification zone beyond that at which the low purity oxygen is withdrawn.

WOLCOTT DENNIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,510,178 Lachmann Sept. 30, 1924 2,256,421 Borchardt Sept. 16, 1941 

1. THE METHOD OF SEPARATING AND RECOVERING SIMULTANEOUSLY HIGH AND RELATIVELY LOW PURITY OXYGEN BY LIQUEFACTION AND RECTIFICATION OF AIR WHICH COMPRISES COMPRESSING A MAJOR PORTION AND A MINOR PORTION OF THE AIR TO BE TREATED, COOLING THE MAJOR PORTION BY HEAT TRANSFER TO GASEOUS PRODUCTS OF THE SEPARATION, SUBJECTING THE MAJOR PORTION OF THE AIR TO PARTIAL LIQUEFACTION TO AFFORD A LIQUID FRACTION CONTAINING IMPURITIES OF THE AIR, COOLING THE MINOR PORTION OF THE BY HEAT EXCHANGE WITH THE HIGH PURITY OXYGEN PRODUCT OF THE SEPARATION, COMBINING AT LEAST A PART OF THE UNLIQUEFIED AND PURIFIED MAJOR POR- 